“It is too soon to speculate” on why the numbers climbed, the agency said. “It should be noted that the historic downward trend in traffic fatalities in the past several years means any comparison will be to an unprecedented low baseline figure,” it added.

Nearly half of the increase in deaths came in the first quarter of 2012, when the death toll was 12.6 percent higher than a year earlier. The rate of increases dropped every quarter and, by the final quarter, it was just 1.7 percent higher than a year before.

The New England region had the greatest rise in traffic deaths, with more than a 15 percent increase. A tier of states mostly in the Southwest (but also including Hawaii, Louisiana and Mississippi) also saw significant increases. The preliminary report did not contain state-by-state figures.

Federal researchers estimated Americans drove 9.1 billion more miles last year than in 2011, but that is only an increase of 0.3 percent.